Error-handling middleware signature in Express - Practice Problems & Coding Challenges

Challenge - 5 Problems

📝 Syntax

intermediate

Identify the correct error-handling middleware signature in Express

Which of the following function signatures correctly defines an error-handling middleware in Express?

A (err, req, res, next) => { /* handle error */ }

B (req, res, next) => { /* handle error */ }

C (err, req, res) => { /* handle error */ }

D (req, res) => { /* handle error */ }

💡 Hint

Error-handling middleware in Express always has four parameters, starting with the error object.

Express error-handling middleware must have four parameters: err, req, res, and next. This signature tells Express to treat it as an error handler.

❓ component_behavior

intermediate

What happens when an error is passed to next() in Express?

In Express, if you call next(err) inside a middleware, what will Express do next?

A Restart the middleware stack from the beginning

B Continue to the next regular middleware ignoring the error

C Skip all remaining non-error middleware and call the next error-handling middleware

D Stop processing and send a default error response immediately

💡 Hint

Think about how Express knows to handle errors differently.

Calling next(err) tells Express to skip all normal middleware and jump to the next error-handling middleware (with four parameters).

🔧 Debug

advanced

Find the error in this Express error-handling middleware

What error will this middleware cause when an error occurs? function errorHandler(req, res, next) { res.status(500).send('Something broke!'); }

Express

function errorHandler(req, res, next) {
  res.status(500).send('Something broke!');
}

A It will not be recognized as error-handling middleware because it lacks the 'err' parameter

B It will cause a runtime error because 'next' is unused

C It will send a 404 status instead of 500

D It will cause a syntax error due to missing parentheses

💡 Hint

Error-handling middleware must have four parameters, starting with the error object.

Express identifies error-handling middleware by the presence of four parameters: err, req, res, next. This function only has three, so Express treats it as normal middleware and will not call it on errors.

❓ state_output

advanced

What is the response status code sent by this error-handling middleware?

Given this Express error-handling middleware, what status code will the client receive? function errorHandler(err, req, res, next) { if (err.status) { res.status(err.status).send(err.message); } else { res.status(500).send('Internal Server Error'); } }

Express

function errorHandler(err, req, res, next) {
  if (err.status) {
    res.status(err.status).send(err.message);
  } else {
    res.status(500).send('Internal Server Error');
  }
}

A Throws an error because res.status is called twice

B Always 200 regardless of error

C Always 404 regardless of error

D 500 if err.status is undefined, otherwise err.status

💡 Hint

Check how the status code is set depending on the error object.

The middleware checks if err.status exists. If yes, it uses that status code; otherwise, it defaults to 500.

🧠 Conceptual

expert

Why must error-handling middleware have four parameters in Express?

Why does Express require error-handling middleware to have exactly four parameters: (err, req, res, next)?

A Because JavaScript requires four parameters for error handling functions

B Because Express uses the function's parameter count to distinguish error handlers from regular middleware

C Because the 'next' parameter is mandatory for all middleware, including error handlers

D Because the 'err' parameter is optional and only used in some error handlers

💡 Hint

Think about how Express decides which middleware to call on errors.

Express checks the number of parameters in middleware functions. Functions with four parameters are treated as error handlers. This is how Express knows to call them when an error occurs.